Not that anybody asked, but…
Measure the circumference of your head in inches, then divide that number by Pi. That’s your Hat size.
Mathematically, your Hat size is the diameter of your head in inches if your head were a perfect sphere.
FYI: Currency. Nutrition labels. Medicine doses. Photography. Electricity, Energy. Piston engine displacement. Track & Field distances. Science. Bottles of water, wine, soft-drinks. Cocaine. All metric in the USA.
Outdoor temperature on Earth not quite your liking today? You’ll freeze solid or vaporize anywhere else. Average temps elsewhere: Mars: -80 F; Venus: +850 F; Moon: day +250 F / night -250 F; Mercury: day +800 F / night -300 F; Pluto: -350 F; Sun: day +10,000 F / night +10,000 F